1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In particular, the invention concerns an improved cascade-type liquefied natural gas (LNG) facility employing additional refrigeration levels in the heat exchanging economizers of one or both of the ethylene and methane refrigeration cycles, thereby enhancing thermodynamic efficiencies without significant additional capital cost.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Currently, the methane refrigeration systems of cascade-type LNG processes are designed so that the natural gas feed stream leaves the ethylene cooling system as a subcooled liquid and enters the methane system for further subcooling. The feed stream is subcooled by the vapors generated from lower-stage flashes and is then expanded into a high-pressure flash drum. The stream changes from a subcooled liquid stream to a liquid/vapor mixture at lower pressure. The vapor phase is returned through the methane economizer where it extracts heat from the predominantly methane feed and is ultimately directed to the methane compressor for recompression. The liquid fraction leaves the high-stage flash drum and enters another economizer stage where it transfers heat to lower-stage flash vapors. The stream is then expanded via an expansion valve into an intermediate-stage flash drum where the stream changes from a subcooled liquid to a vapor/liquid mixture which is separated in the intermediate-stage flash drum. The vapor leaves the intermediate-stage flash drum and is directed through the economizers wherein it extracts heat from the processed natural gas feed and is ultimately recompressed in the methane compressor. The liquid leaving the intermediate-stage flash drum is then expanded and introduced into a low-stage flash drum. The vapor leaves the low-stage flash drum and passes through the economizers, where it extracts heat from the processed natural feed and is then recompressed. The liquid leaves the low-stage flash drum to LNG storage.
In the ethylene refrigeration systems of conventional LNG processes, the ethylene refrigerant is condensed in the propane refrigeration system. Thereafter, the ethylene is subcooled in the ethylene economizer and is then expanded into the high-stage ethylene chiller where it is used to cool the natural gas feed.
Designers of LNG processes are constantly seeking ways to improve the thermodynamic efficiencies of the systems. While in theory this can be accomplished by providing additional refrigeration capacity, there is a point of diminishing returns where the capital costs associated with the added capacity are greater than the return. Therefore, the goal is to have maximum thermodynamic efficiencies coupled with the lowest possible costs.